This invention relates generally to semiconductor materials with novel properties and internal structures. In particular, the invention relates to a non-single crystalline semiconductor, and a process for making same, comprised of coordinatively irregular structures, each of which has distorted chemical bonding, reduced dimensionality, and a state of structural order distinct from the amorphous and single crystalline forms of the semiconductor.
Semiconductor materials have had a tremendous impact on the quality of life and economic progress across the world over the past four decades. Semiconductor materials are integral to modern electronic devices such as transistors, diodes, LEDs, and lasers. These and related devices are responsible for the advent of the information age with all of its promise for today and tomorrow.
First generation electronic devices were based primarily on crystalline silicon. Single crystalline silicon, especially when doped to produce n-type and p-type material, was and continues to be successfully used in a variety of electronic devices. The efficacy of crystalline silicon is due to its high charge carrier mobility and suitability for high speed electronics applications.
The primary limitations of single crystalline silicon for photovoltaic applications are its indirect bandgap and the inability to produce it in large areas in a continuous manufacturing process. The indirect bandgap of crystalline silicon has two important deleterious consequences. First, optical transitions from the valence band to the conduction band of crystalline silicon occur with weak intensity and second, crystalline silicon is unable to emit light. As a result, crystalline silicon is impractical for many optical and photonic applications.
Although single crystalline silicon can be prepared with a high degree of purity and with well-controlled spatial distributions of n and p type dopants, its preparation is slow and not amenable to high speed manufacturing processes. Consequently, the preparation of single crystalline silicon is expensive and cost considerations limit its range of applications.
The need for an optically efficient semiconductor material for laser, LED, solar energy and photovoltaic applications has motivated much research over the years. Direct gap III-V materials such as GaAs and InP exhibit strong optical transitions and have been shown to be effective light emitting and absorbing materials. These materials, however, are only practically useful in the single crystalline state and are subject to many of the same processing and cost constraints associated with crystalline silicon. Since III-V materials and alloys are compound semiconductors, their preparation is further complicated by the need for a proper stoichiometric ratio of two or more elements. The need for uniform chemical composition imposes additional restrictions on the preparation and processing of III-V materials and alloys. Consequently, III-V materials and alloys are currently limited to niche applications.
Amorphous silicon has emerged as the leading material for large scale solar energy and photovoltaic applications. Amorphous silicon is an unusual material in that although it is silicon based, it possesses a direct bandgap and therefore exhibits high absorption efficiency. Since the bandgap of amorphous silicon occurs in the visible part of the spectrum, it has been demonstrated to be an effective material for solar cells and other photovoltaic devices capable of being powered by the sun. The amorphous nature of amorphous silicon precludes the need to establish the structural regularity associated with single crystalline silicon. As a result, the growth rate of amorphous silicon is much faster than that for single crystalline silicon and amorphous silicon can be prepared on a large scale by a variety of deposition techniques in a rapid, continuous and cost-effective manner.
S. R. Ovshinsky recognized that the disordered nature of amorphous materials provides new opportunities for tailoring electronic properties. S. R. Ovshinsky believed that crystalline solids are restrictive in terms of their properties because of the limited number of structures available and the limited flexibility in achieving new chemical compositions. These limitations are inherent to the unforgiving periodic and ordered structural requirements of crystalline solids. By embracing the structural disorder of the amorphous state, S. R. Ovshinsky argued, it becomes possible to achieve new structures and new compositions with new electronic properties.
S. R. Ovshinsky further showed that it was possible to prepare materials that included features of both the amorphous and single crystalline forms of the composition. These materials typically comprise an amorphous or crystalline matrix that contains regions of ordered clusters or aggregations of atoms with a degree of order intermediate between the highly ordered single crystalline form and the highly disordered amorphous form. The presence of the clusters or intermediate range order aggregations imparts unusual electronic properties to the material and has motivated further development of this fundamentally new class of materials. This seminal concept of achieving new materials with superior electronic properties by varying the degree of order through atomic engineering has been described in, for example, xe2x80x9cAmorphous and Disordered Materialsxe2x80x94The Basis of New Industriesxe2x80x9d, S. R. Ovshinsky, Materials Research Society Symposium Proceedings, Vol. 554, pp. 399-412, 1999; xe2x80x9cHeterogeneity in Hydrogenated Silicon: Evidence for Intermediately Ordered Chainlike Objectsxe2x80x9d, D. V. Tsu et al., Physical Review B, Vol. 63, pp. 125338: 1-9, 2001; xe2x80x9cSemiconductor with Ordered Clustersxe2x80x9d, S. R. Ovshinsky et al., U.S. Pat. No. 6,087,580; and xe2x80x9cSemiconductor having Large Volume Fraction of Intermediate Range Order Materialxe2x80x9d, S. R. Ovshinsky et al., U.S. Pat. No. 5,103,284.
From the viewpoint of solar energy applications, amorphous silicon is not an optimal material because it does not absorb the full range of photon energies present in the solar spectrum. Since amorphous silicon has a bandgap energy of approximately 1.8 eV, it is only capable of efficiently absorbing light with photon energies greater than about 1.8 eV. (The UV and higher energy visible portions of the solar spectrum.) The solar spectrum, however, contains a significant amount of light with photon energies less than 1.8 eV. (The lower energy visible and infrared portions of the solar spectrum.) As a result, solar energy devices incorporating only amorphous silicon capture only a limited fraction of the total energy available from sunlight.
In order to increase the amount of sunlight collected, practical solar energy devices are normally based on multilayer structures comprised of amorphous silicon to capture the high photon energy portion of the solar spectrum and an alloy of amorphous silicon with a bandgap narrowing element to capture the low photon energy portion of the solar spectrum. A bandgap narrowing element is an element that leads to a reduction in the bandgap energy and consequently an increased absorption of lower photon energy light. Bandgap narrowing elements preferably lower the bandgap while maintaining a direct bandgap so that the resulting alloy material retains the high absorption strength characteristic of amorphous silicon. Germanium is the most commonly used bandgap narrowing element. Alloys of silicon and germanium are capable of strongly absorbing the low energy visible and infrared portions of the solar spectrum and lead to substantial improvements in sunlight-to-electricity conversion efficiency when incorporated into solar energy devices.
Although the incorporation of germanium in solar energy devices improves device performance, its inclusion has two disadvantages. First, the most common source of germanium, germane gas (GeH4), is expensive and not widely available. Second, incorporation of germanium adds complexity to the process used to manufacture solar energy and photovoltaic devices. Additional processing units are needed to supply and deposit germanium during manufacturing. These units add cost and time to the manufacturing process.
Based on the prior art, it would be desirable to have a material, that does not include Ge, with sufficiently strong absorbance in the red and/or near-infrared to be useful in solar energy and photovoltaic devices. A material capable of absorbing as many of the photon energies of the solar spectrum not absorbed by amorphous silicon is preferred. Ideally, the material should be readily integrable with amorphous silicon to expedite and economize the processing of multilayer structures. The material itself should also be inexpensive and readily available.
There is disclosed herein a non-single crystalline semiconductor material comprised of silicon and a process for making same. The material is an assembly of coordinatively irregular structures, each of which has a state of structural order and bonding configuration distinct from the amorphous and single crystalline forms of the semiconductor. The electronic properties of each constituent coordinatively irregular structure are determined by its state of structural order, coordination properties and bonding configuration. By controlling the size of individual coordinatively irregular structures and the size and spatial distributions of coordinatively irregular structures within a semiconductor body, it is possible to achieve silicon-based materials with desirable new properties. The electronic properties of the semiconductor material of the present invention are desirable in applications such as solar cells, photovoltaic devices, lasers, LEDs, transistors, and diodes. In one embodiment, for example, the material of the present invention is silicon-based and is incorporated as the bottom layer of a triple junction solar cell thereby making it possible to efficiently collect the long wavelength portion of the solar spectrum without using germanium or other bandgap narrowing elements.
The material of the present invention is prepared by repeated application of a two-step process. In a formation step, a sub-coalescent amount of a non-single crystalline silicon material is formed. A sub-coalescent amount is an amount of material that is insufficient to provide full coverage of the surface upon which it is formed. Instead, portions of the surface upon which formation occurs remain uncovered. As a result, the sub-coalescent structures that are formed in the formation step contain a high fraction of surface atoms and possesses a distorted bonding configuration. The purpose of the formation step is to provide sub-coalescent structures that will subsequently be stabilized and incorporated into the body of the ultimate semiconductor to be formed. In the treatment step, the sub-coalescent structures formed in the formation step are treated with a plasma comprised of hydrogen, fluorine or a combination of hydrogen and fluorine in a suitable reaction chamber. The purpose of the second step is to terminate the coordinatively unsaturated surface atoms of the sub-coalescent structures to produce and stabilize coordinatively irregular structures that are characterized by a state of structural order, coordination properties and chemical bonding that are distinct from those of the amorphous and crystalline phases with the same composition.
Repeated application of the two processing steps leads to the preparation of an ultimate semiconductor body comprised of an assembly of individual coordinatively irregular structures. Since the constituent coordinatively irregular structures are characterized by unconventional structural order, coordination properties and bonding configurations, they individually possess unusual properties and when collectively assembled, lead to new semiconductor materials with heretofore unobserved properties.